EP1671405A1

EP1671405A1 - Spark plug

Info

Publication number: EP1671405A1
Application number: EP04743145A
Authority: EP
Inventors: John Anthony Burrows; Simon Freeman
Original assignee: Federal Mogul Ignition UK Ltd
Current assignee: Federal Mogul Ignition UK Ltd
Priority date: 2003-07-29
Filing date: 2004-06-30
Publication date: 2006-06-21
Anticipated expiration: 2024-06-30
Also published as: GB2404422A; GB2404422B; EP1671405B1; DE602004003972D1; GB0317671D0; WO2005015702A1; DE602004003972T2

Abstract

A spark plug (100, Fig 2) for an internal combustion engine comprises a tubular metallic shell (120) defining an axial counterbore along which extends an electrically insulating sleeve (12) surrounding a first electrode (16) that cooperates with a second, ground electrode (22) carried by the shell to define a primary spark gap G1. The insulating sleeve carries one or more lateral projections (32) of electrically insulating material spaced from the tip and the metallic shell part extends axially to the vicinity of the projection as a plurality of shell extension elements 1211, 1212 of limited circumferential extent separated by spaces 1231, (123)2 that define lateral sleeve exposing openings. The extension elements define with the sleeve projection secondary spark gaps G2 and the openings permit fuel to access the counterbore in the vicinity of the secondary spark gaps and a flame front of fuel ignited by a spark at a secondary gap to pass efficiently into the combustion chamber. The limited exposure of the sleeve to the combustion chamber by the opening spaces and accessibility of the counterbore ensures less fouling and better cooling of the sleeve that enables it to function at higher engine temperatures. The extension elements may be capped by a circumferentially extending ring (325, Fig 4) or the elements and intervening spaces may be defined by a tubular wall with through apertures (427, Fig 5).

Description

Spark Plug

This invention relates to spark plugs for igniting fuel in internal combustion engines and in particular relates to preventing ignition malfunctions caused by failure of such a spark plug to effect ignition at the proper time.

Publication WO 03/0453152 (GB-A-2382842) sets out various spark plug constructions that have in common an electrically insulating sleeve extending along a central axis, a first electrode mounted within the sleeve with a tip extending beyond an end portion of the sleeve, and a ground conductor extending alongside the sleeve with a clearance therebetween. The ground conductor comprises in combination a metallic shell part surrounding the sleeve, and by which the plug is mounted physically and electrically with respect to a cylinder head of the engine, and, mounted on the end of the shell, at least one second electrode having at least one electrode tip disposed adjacent the first electrode to define a primary spark gap across which fuel-igniting sparks are generated in use. The shell provides a circumferentially continuous counterbore that shrouds the sleeve other than at the end portion that then extends by way of the end of the shell into contact with vaporised fuel in the cylinder.

As is known in the art, any particular engine experiences a variety of different operating conditions that place conflicting demands upon the spark plug each requiring different electrical, thermal and mechanical attributes, such that its physical construction is usually a compromise.

For example, if the end of the insulating sleeve extends too far into the cylinder combustion chamber and is exposed to the extreme temperatures of combustion, it may, as a poor thermal conductor unable to dissipate the heat, reach temperatures at which fuel prematurely ignites before a spark is generated. It may also degrade structurally and shorten the useful life of the plug. However, if the end portion of the sleeve is insufficiently inserted into the cylinder there is the possibility, particularly when an engine is cold, of contaminants or fuel being deposited on the insulating sleeve and providing a conductive path having less impedance that the primary spark gap by which the spark energy tracks to the shell and ground instead of the primary spark gap. If the spark energy reaches the shell without jumping the primary spark gap or the counterbore clearance the plug will become progressively more fouled and fail to work at all, whereas if a spark discharges the energy to ground, whether by jumping the primary gap or a secondary gap provided by the counterbore clearance and initiates fuel combustion, the heat and pressure of such combustion extends into the counterbore to clean the surface of the insulating sleeve and promote proper discharge across the primary spark gap subsequently.

Spark plug development has recognised the importance of such discharge across a secondary spark gap and its success being dependent upon it being able to effect ignition of the fuel charge at the end of the plug, that is, to ensure that a secondary spark gap is accessible to fuel vapours by ensuring that any secondary spark gap is formed at the interface between the plug counterbore and combustion chamber and effecting movement of combustion chamber contents into and out of the counterbore during the various pressure changes found during the engine cycle. Such latter effect not only ensures that sparking across a secondary gap takes place in a fuel igniting environment but also that the combustion gases at other times in the engine cycle are less likely to form deposits on the insulating sleeve.

In traditional designs, constraints in the formation of such secondary spark gaps that ensure fuel ignition ensues often lead to such secondary spark gaps being formed by supplementing the second, ground electrode of the shell with one or more further ground electrodes that have tips approaching the insulating sleeve where it is exposed adjacent the first electrode tip. Thus whereas discharge across a secondary spark gap results from a discharge tracking across part of the insulating sleeve and jumping the secondary spark gap to ground, and should have a higher sparking threshold than the primary spark gap, the requirement for it to offer less impedance than tracking over the insulating sleeve to ground may demand a smaller gap than the primary spark gap. Whilst any spark should form preferentially at the primary spark gap because of the absence of the tracking impedance of the insulating sleeve, as the location a secondary spark gap may involve minimal tracking distance in addition to a smaller gap, a spark may be triggered to form at the secondary spark gap in preference to the primary spark gap for little if any contamination of the sleeve; provided that fuel combustion ensues this is not necessarily considered to be an operational failure, but it does require the metal electrodes associated with such secondary spark gap or gaps to be of alloys that are resistant to spark erosion and therefore expensive. The aforementioned WO 03/0453152 describes a variety of such traditional designs but additionally describes various embodiments of an improved spark plug construction that provides the insulating sleeve with one or more lateral projections of electrically insulating material at its end portion, defining a reduced clearance between the insulating sleeve surface and the end of the conductive shell that surrounds the sleeve whereby a discharge from the first electrode tip that tracks across the surface of the insulating sleeve instead of striking a spark at the primary spark gap tracks across a projection which concentrates the electric field at a position of reduced clearance distance to the shell and discharges as a spark across the gap thereto in preference to elsewhere; that is, the or at least one sleeve projection and shell end define a secondary spark gap which is in contact with the combustion chamber to ensure ignition of fuel therein. Whereas formation of such a secondary spark gap from the insulating material of the sleeve permits it to be manufactured economically (moulded integrally with or attached to the sleeve) and permits it to be made directionally independent (as an annular rib or array of projections), the use of such sleeve projection or projections is particularly attractive as it permits the tracking distance from the first electrode to any projection peak and the associated secondary spark gap to be longer than the tracking distance formed by mounting a further ground electrode tip in close proximity to a conventional insulating sleeve nose, thus increasing the trigger threshold of a secondary spark, whilst permitting a steep sided projection which has a peak that is dimensionally small in at least one direction to effect an electrical field concentration suited to localised spark formation.

Although this design approach provides a simple and economic plug construction that is an improvement over earlier designs, experience of such plug design in engine operations has revealed that there are still situations where there is room for further improvement in performance.

It is an object of the present invention to provide an internal combustion engine spark plug that mitigates some of the shortcomings identified with existing plug designs in a simple and economic manner. According to the present invention a spark plug comprises an electrically insulating sleeve extending along a central axis of the plug, a first electrode mounted within the sleeve and having a tip extending beyond an end portion of the sleeve, a ground conductor, extending alongside the sleeve with a clearance therebetween, comprising a metallic shell part surrounding the sleeve axially spaced from the end portion of the sleeve to provide a circumferentially continuous shrouding thereof and a second electrode connected to the shell having at least one second electrode tip disposed adjacent the first electrode to define a primary spark gap, and at least one projection formed from electrically insulating material projecting laterally from the sleeve end portion so that the clearance between the sleeve and ground conductor alongside is locally reduced in an axially limited, electrical charge concentrating manner that effects formation of at least one secondary spark gap between the insulating sleeve and the ground conductor, the plug being characterised by said ground conductor extending axially from the circumferentially continuous shell part in a direction towards the end of the sleeve and providing in the vicinity of said sleeve projection or projections circumferentially alternate regions of ground conductor and lateral sleeve exposure opening, at least one of said ground conductor regions creating with at least one said sleeve projection a circumferentially limited secondary spark gap adjacent a said lateral sleeve exposing opening.

Preferably, the ground conductor extends axially as a plurality of shell part extension elements each of limited circumferential extent separated by spaces therebetween defining the lateral sleeve exposure openings. Furthermore it is preferred that the sleeve exposure opening spaces have circumferential width not less than the radial thickness of the adjacent shell part extension elements, at least in the vicinity of the sleeve projection.

The shell part extension elements may be formed integrally with the circumferentially continuous shell part or discretely and mounted thereon. The extension elements may have their ends remotely from the shell part unsupported or mutually supported thereat. Such mutual support may be provided by forming the elements from a circumferentially complete skirt wall that is apertured to provide the sleeve exposing opening spaces and shell part extension elements between them. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figures 1(a) and 1(b) are sectional elevation views through different forms of spark plug from the prior art and constructed in accordance with WO 03/0453152, Figure 2 is a perspective view of an end portion of a first embodiment of improved spark plug in accordance with the present invention, showing a first electrode tip in an insulating sleeve, a ground conductor as a metallic shell surrounding all but an end portion of the insulating sleeve having a projection in the form of an annular rib, the shell part having integral axial extension elements to the vicinity of the projection rib separated from each other circumferentially by sleeve exposing openings, and a second electrode associated with the first electrode tip mounted on one of the axial extension elements of the shell part,

Figure 3 is a perspective view of an end portion of a second embodiment of spark plug in accordance with the present invention, generally similar to that of Figure 2 but wherein the axial extension elements of the shell part are discrete electrodes mounted to the end of the shell part, one of the axial extension elements forming the second electrode of the plug associated with the tip of the first electrode and forming therewith the primary spark gap,

Figure 4 is a perspective view of an end portion of a third embodiment of spark plug in accordance with the present invention, generally similar to that of Figure 2 except that the ends of the axial extension elements remote from the shell part are mutually supported by a ring member,

Figure 5 is a perspective view of an end portion of a fourth embodiment of spark plug in accordance with the present invention, in which the shell part is extended axially as a circumferentially complete skirt wall made discontinuous by an array of through apertures therein exposing the part of the insulating sleeve carrying the insulating projection rib,

Figures 6(a) to 6(f) are a non-exhaustive series of schematic views of possible shapes for the axial extension elements of Figures 2 to 4 in terms of length in the axial direction and width in the circumferential direction,

Figures 7(a) to 7 (h) are a non-exhaustive series of schematic views of possible shapes for the axial extension elements of Figures 2 to 4 in terms of length in the axial direction and thickness radially, and

Figure 8 is a graphical representation of relative performance of thermal properties of the spark plug of Figure 2 and prior art plug of Figure 1 (b). Referring to Figure 1(a) this represents a portion of a metallic cylinder head 8 of an internal combustion engine and a combustion chamber 9 adjacent thereto, and a spark plug 30 of a form shown and described in detail on the aforementioned WO 03/0453152, mounted in the head so as to extend for operation into the combustion chamber; the plug 30 will be described here briefly to assist in appreciation of the present invention.

The plug 30 comprises a sleeve 12 of electrically insulating material, such as an alumina based material, extending along a central axis 14 of the plug and a first electrode 16 mounted within the sleeve and having a tip 16a extending beyond a sleeve end portion 12c. The plug further comprises a ground conductor, indicated generally at 17, which extends alongside the sleeve with a clearance 19 therebetween.

The ground conductor comprises a metallic shell part 20 having an external thread 20a, by which the plug is mounted in the head 8 physically and electrically, and an end portion 20c which terminates in a transverse, radially extending end face 20b , the end portion at least of the shell part having a generally cylindrical internal surface 20d surrounding the sleeve axially spaced from the end portion 12c of the sleeve to provide a circumferenatially continuous shrouding thereof. The ground conductor 17 also comprises a second electrode 22 mounted on, and electrically connected to, the shell 20 at the end surface 20b, projecting in a J-shape to a tip 22a thereof in opposed relationship with the tip 16a of the first electrode and defines therewith a primary spark gap G1. As will be apparent from the Figure, the end portion 12c of the sleeve, the first electrode tip 16a and the second electrode 22 all extend for a short distance into the combustion chamber 9 and the annular clearance between the shell surface 20d in the counterbore of the shell and the remainder of the shrouded sleeve part is exposed to the combustion chamber by way of the end of the shell.

The plug 30 departs from conventional construction by having at least one projection formed from electrically insulating material projecting laterally from the sleeve end portion, conveniently as an annular rib 32 integral with the sleeve, so that the clearance between the sleeve and ground conductor alongside is basically reduced in an axially limited manner that effects formation of a secondary spark gap between the ground conductor and the insulating sleeve when the latter is contaminated by an electrically conductive coating. The sleeve projection rib 32 is disposed adjacent the end face 20b of the shell so that the projection defines with the shell an annular, and thus omnidirectional, secondary spark gap of separation G2 and at any part of which a spark may jump between the sleeve and ground. The radial clearance between the sleeve and shell at the gap G2 is less than it is within the shell, so that if a spark fails to form in primary gap G1 and a discharge tracks along the, contaminated surface of the sleeve, a spark is encouraged to jump the gap G2 exposed to the combustion chamber in preference to the discharge tracking further along the shell.

Formation of the one or more projections 32 on the insulating sleeve enables the creation of a secondary spark gap which exists only when the surface of the sleeve is contaminated and conductive, and in forming it as an annular rib, provides a symmetry about the central axis that simplifies manufacture. Furthermore, notwithstanding the circumferential extent of the or each projection, the ability to mould each such projection from the insulating material enables it to be radially formed at least in the axial direction with at least one steeply sloping side or otherwise effect a rapid change in sleeve surface level to form an electric charge concentrating region with the adjacent shell part that defines an effective secondary spark gap.

Referring also to Figure 1(b), this shows an alternative spark plug 30₂ that is generally similar to the plug 30 except that the corresponding shell 20₂ extends further into the combustion chamber as an un-threaded annular skirt 20₂s on which the end face 20₂b is formed and projection rib 32₂ is formed close to the end of the insulating sleeve in the plane of the shell end face 20b, the projection again being in the form of an annular rib. Optionally in this. construction, or indeed that of plug 30, the sleeve projection 32₂ may be accompanied by one or more further, and possibly less pronounced, projections, conveniently as ribs 32₂\ 32₂", ... shown ghosted. Such a plurality of projections provides a longer tracking path to the shell for a failed spark discharge at the secondary gap, that is, make the alternative to discharge at a spark gap less likely. In this plug form, notwithstanding the presence of optional secondary projections 32₂' etc, the secondary spark gap G2 is provided deeper into the combustion chamber whilst shielding much of the insulating sleeve from excessive heat of the combustion chamber.

Whereas such constructions provide improvements over other plug designs, particularly for cold running, it has been found that creating such an annular secondary spark gap at the end of the plug counterbore by reducing the clearance may result in a disappointing level of improvement to both cold and hot running.

Referring now to Figure 2 which shows an end region only of a spark plug 100 according to a first embodiment of the present invention, many of the parts thereof correspond to those of the plugs 30 and 30₂ mentioned above and will be given like reference numerals to avoid repetition of description. Shown in the Figure are electrically insulating sleeve 12 extending along central axis 14, first electrode 16 with tip 16a, and ground conductor 117. The ground conductor 117 takes the form of a second electrode 22, having tip 22a disposed overlying the first electrode tip and defining therewith primary spark gap G1 , and threaded metallic shell part 120 surrounding all but an end portion 12c of the sleeve that includes, adjacent the end and tip 16a thereat, a lateral projection 32. In this embodiment there is a single sleeve projection that extends circumferentially around the sleeve in the form of an annular rib. The rib is of electrically insulating material, conveniently the same as the sleeve and formed integrally therewith, although it may be formed separately of the same or different material. The end portion of the sleeve extends from the threaded part of the shell for such distance that it extends significantly into the combustion chamber, in the manner of plug 30₂.

The shell part 120 also extends axially in a direction towards the end of the sleeve to provide in the vicinity of the sleeve projection circumferentially alternate regions of ground conductor and lateral sleeve exposure so that at least one, and preferably all, of the ground conductor regions create with the sleeve projection a secondary spark gap adjacent a said region of sleeve exposure.

The shell part 120 extends as a plurality of shell part extension elements 121-₁, 121₂,

121₃, ... each of limited circumferential extent and separated from each other by gaps or spaces 123^ 123₂, ... therebetween defining the lateral regions of sleeve exposure. The extension elements 121_! etc. are integral with the circumferentially continuous shell part and extend substantially to the axial position of the projection rib 32.

Circumferentially alternate extension elements and opening spaces may be formed conveniently by manufacturing the shell part with an axially extending, circumferentially continuous skirt wall and thereafter removing parts of the wall to define the spaces.

The shell extension elements are conveniently, but not necessarily, identical, having the same dimensions as each other of length in an axial direction, thickness in a radial direction, and width in a circumferential direction.

Insofar as each extension element conveniently is of a thickness corresponding to that of the circumferentially continuous shell part, the clearance between the elements and sleeve body is a continuation of the clearance formed with the shell counterbore other than in the vicinity of the sleeve projection rib, where the clearance is reduced to effect for each extension element a secondary spark gap G2, and insofar as each shell extension element is formed from the shell it is arcuate and spaced from the sleeve projection rib 32 with a substantially constant clearance that defines an associated secondary spark gap.

Preferably, in the manner of the aforementioned plugs 30 and 30₂, but less essentially as will become apparent, the length of each shell part extension element is such that it terminates in the plane of the ridge of the sleeve projection so that the edge or corner between the wall and end face of the element is in line with the ridge of the sleeve projection to maximise electric field concentration and spark formation at a position that gives the fuel flame front arising from such spark ready access to the combustion chamber from the end of the plug. However, as the separated extension elements have axially extending edges that also contribute to spark formation there is certainty of spark formation at any secondary spark gap even absent axial alignment of the end of the extension elements and sleeve projection peak.

The number of shell part extension elements and circumferential extent of each is a matter of choice insofar as it is desirable to provide secondary spark gaps at many circumferential positions and to have dimensional limiting of each spark gap to improve electrical charge concentrations. Notwithstanding the effect of limiting the circumferential extent of any extension element in terms of spark generation, the secondary spark gaps are not the only consideration and the dimensions of the shell extension elements and the intervening opening spaces are important insofar as the spaces permit exposure of the sleeve and the secondary spark gaps to the cylinder contents that has electrical, thermal and mechanical effects.

The principal objective is to ensure that fuel of the combustion chamber is present in the vicinity of a secondary spark gap and that having ignited fuel by generation of a spark thereat, the flame front of the ignited fuel is not unobstructed or occluded by the presence of the extension elements from propagating into the combustion chamber, and to this end, a spark from any of these secondary spark gaps is able to effect ignition of fuel within the combustion chamber by way of the opening spaces adjacent extension elements as well as the end of the plug.

There is no specific minimum separation of extension elements, that is, circumferential extent of any intervening opening space, except that it must not unduly interfere with flame front propagation. For this embodiment wherein the extension elements are continuations of the shell part wall and of corresponding thickness, a practicable limitation for each opening space is to have a circumferential width not less than the radial thickness of the adjoining extension elements (wall) so that the elements bounding each opening space do not mask or occlude the ignition from the chamber and impede passage of the flame front. It will be appreciated that the thickness of the extension elements may vary and also the opening spaces between them may vary in circumferential width throughout the thickness of the bounding elements. For example, if a tubular shell wall extension is cut with radially directed cuts and material removed to define the opening spaces, the boundaries of the elements will diverge and the spaces will increase in circumferential width in a radially outward direction. In terms of flame propagation from within the space surrounded by the extension elements this is a good thing and preferably the circumferential width of each opening space is such that its maximum width is also greater than the thickness of the bounding extension elements. In practise the opening spaces may be formed with an exaggerated divergence in the radial direction to maximise ignition exposure to the combustion chamber. There is no maximum limit to the circumferential width of each opening space except that the complementary minimising of the circumferential width of each extension element place increasingly impracticable constraints on manufacture and shortening of operating life from the effects of spark erosion. In tests, it has been found that a spark plug having three opening spaces between three extension elements that subtend angles at the central axis of about 20 degrees and 100 degrees, respectively, performs well.

Because each of the secondary spark gaps G2 may be implemented with greater definition and precision, and be caused to spark efficiently but much less frequently than the primary spark gap, the shell extension elements which define the secondary spark gap may readily be formed from the base metal used for the sleeve part itself rather than from expensive alloys resistant to spark erosion by frequent discharges, although that is not excluded as a mater of choice.

The opening spaces between the extension elements not only permit propagation of an ignited fuel flame front from any secondary spark gap but also permit heat and pressure within the cylinder to impact upon the sleeve and shell counterbore to effect cleaning and, during other parts of the engine cycle, the sleeve to be exposed to gas and fuel mixture flow that effects heat extraction, although the extent to which the sleeve is exposed to, and retains the heat of combustion is influenced by the shell part extensions also having some shielding effect on the sleeve.

Thus the relationship between the numbers and circumferential extents of the opening spaces and shell extension elements determines the thermal behaviour of the plug in use and subject to other constraints on their variability these dimensions may be selected to permit an increase in operating temperature of an engine it is in.

Referring also to Figure 8, as mentioned in the introduction it is known that prolonged exposure to excessive temperatures of the insulating sleeve of a spark plug gives rise to extraneous ignition of the surrounding fuel and it is found that a plug according to the invention is less prone to said undesired ignition for any particular engine temperature, that is, effectively able to run cooler, and able to tolerate significantly higher engine operating temperatures before producing the same degree of undesirable ignition. It may be seen from Figure 8 that for a plug 30₂ and the plug 100 in a particular test engine, when the engine operating temperature was raised in stages and the percentage of undesirable ignitions for each determined, the plug 100 in accordance with this invention exhibited the ability to perform safely at engine ignition timings 10 -15 degrees more advanced than the prior plug.

It will be appreciated that there are a number of other variables that may be changed individually or together.

It is possible for the extension elements to be of such length axially that they terminate beyond the plane of the sleeve projection or projections, whereby the secondary spark gaps are formed predominantly at a much reduced number of locations circumferentially defined by the axially extending edges of the sleeve extension elements and flame front propagation is predominantly by way of the opening spaces between the elements.

Insofar as it is normal to mount the second electrode 22 to the end of the shell part, it is shown mounted to the end of one of the shell part extension elements (121₃). It will be appreciated that it may alternatively be mounted to the shell part in one of the spaces between extension elements. Depending upon the radial thickness dimension of the second electrode thus mounted, it may form with the sleeve projections a secondary spark gap.

Just as the second electrode is formed discrete from, and mounted to, the shell part, then as shown in Figure 3, a second embodiment 200 of spark plug may have a ground conductor 217 including a circumferentially continuous shell part 220 having shell extension elements 221₁, 221₂, ... formed as discrete elements of limited circumferential width mounted to the end of shell part 220 with spaces 223^ 223₂, ... therebetween. The extension elements may be of a different material from the shell part, possibly an alloy more resistant to spark erosion and/or a better thermal conductor, and any one may have a cross section that- does not conform to the arcuate shape of the end of the shell part. As mentioned above, the second electrode 22 may be mounted to the end of one of the extension elements or may be mounted directly to the end of the shell part between extension elements or, as shown, instead of one of shell extension elements. Insofar as such a second electrode is able to form a secondary spark gap with the sleeve projection it may be considered as both a discrete shell extension element 221₃ and second electrode.

In the plug embodiments 100 and 200 the shell extension elements are fixed in relation to each other only at the end of the circumferentially continuous shell part, and the distal ends that define the secondary spark gaps are exposed and potentially subject to displacement by impact during handling.

Referring now to Figure 4, a third embodiment of spark plug in accordance to the invention, 300, has ground conductor 317 including circumferentially continuous shell part 320 and axially extending shell part extension elements 321₁, 321₂, 321₃,

..., integral or discrete, which are mutually supported at their distal ends by a ring

325 secured thereto at an axial position in the vicinity of the peak of the sleeve projection or projections. Conveniently the ring is thin (in the axial direction) and disposed to lie in the plane of the annular ridge that forms the projection peak whereby a secondary spark gap is formed that permits a spark discharge between the ridge and one of the ring face boundaries without any constraint on circumferential position and a flame front created thereby to have access to the combustion chamber by way of the open end of the plug or the opening spaces between the supporting shell extension elements.

As discussed above, it is possible for the ends of the extension elements, and thus the ring supported thereby, to be disposed out of the plane of the sleeve projection ridge such that a secondary spark gap is formed with the circumferentially extending bottom edge of the ring or with the axially extending supporting elements, the opening spaces therebetween permitting passage of fuel and flame front between the insulating sleeve and the combustion chamber.

Referring now to Figure 5, this shows at 400 a fourth embodiment of spark plug according to the invention. In this embodiment, the ground conductor 417 extends axially from circumferentially continuous shell part 420 as a circumferentially complete skirt wall 427 that is conveniently integral therewith but may be discrete and mounted thereto. The skirt wall extends axially to the end portion of the sleeve, preferably in the plane of the lateral sleeve projection peak, the edge of the wall at the end face 427a providing with the projection a circumferentially complete secondary spark gap G2.

The skirt wall 427 contains axially and circumferentially bounded through apertures 423! , 423₂, 423₃, ... in the vicinity of the sleeve projections, thereby defining circumferentially alternate regions of shell part extension 421 ^ 421₂, 421₃, ... and lateral sleeve exposure.

A second electrode 22 may be mounted on the end of the extension skirt wall. Alternatively, analogously to the variants described above and not specifically shown, the sleeve 471 may extend beyond the plane of the ridge denoting the projection peak such that secondary spark gaps are in fact formed with the sleeve at the edges -of the through apertures that define the opening spaces, and the apertures provide the opening spaces for passage of the flame front to the combustion chamber and fuel from the chamber for cooling the insulating sleeve and ignition. Depending upon the material of the skirt wall, the end of the skirt wall remote from the shell may be shaped so that it approaches the first electrode tip and defines therewith the first electrode gap overlying or surrounding the tip.

It will be appreciated that although it may be convenient for manufacture to have uniformity in numbers, spacings and dimensions of the shell part extension elements and/or lateral sleeve exposure openings, such uniformity is not essential and variations may be made, some with beneficial effects. For example, one or more of the extension elements may be formed so as to decrease in circumferential width towards the distal end in the vicinity of the sleeve projection or projections so as to reduce the circumferential extent of the associate secondary spark gap and maximise the lateral sleeve exposure thereat without compromising the physical strength of the plug.

Referring to Figures 6(a) to 6(e) these show side views of exemplary extension element shapes, but it will be understood that other shapes may be chosen to suit particular applications. Figure 6(a) shows generally rectangular extension elements as illustrated in Figures 2 to 4 described above, having substantially uniform circumferential width W_ε for the whole axial length L_E, such that the adjacent, and thus intervening, lateral sleeve exposure opening spaces 123ι etc each has a circumferential width W₀ that is also uniform axially. It will be understood that any or all of the dimensions L_E, W_E and W₀ may vary from element to element circumferentially.

Figure 6(b) shows the shell part extension elements having linearly tapered sides and disposed immediately adjacent each other as a zigzag end to the circumferentially continuous shell part. The extension elements thus each have circumferential width W_E that reduces axially from W_Emax at the shell part to nearly zero at the secondary spark gap and the associated, intervening, lateral sleeve exposure opening spaces each have width W₀ that increases axially from zero at the shell part to W₀max adjacent the secondary spark gap. The extension elements may, of course, be spaced apart at the shell part so that the sleeve exposure opening spaces increase in width from greater than zero width.

Figure 6(c) shows a variation of Figure 6(b) wherein the shell extensions have an asymmetrical saw-tooth profile in which one edge extends axially and the opposite edge is inclined to the axial direction to effect tapering.

Figure 6(d) shows extension element forms which represent a further variant of combining the forms of Figures 6(a) and 6(b), namely a symmetrical trapezoidal form

Figure 6(e) illustrates a non-linear tapering form for extension elements which each have a generally sinusoidal profile but with, optionally, lateral sleeve opening spaces having a width W that increases from a non-zero value at the end of the circumferentially continuous shell part to a maximum width at the level of the secondary spark gaps.

Figure 6(f) shows extension element forms similar to those of Figure 6(e) but having a more rounded, dome-like form in the vicinity of the secondary spark gaps.

It will be appreciated that although it may be convenient to provide the shell part extension elements of uniform thickness T along their length, with the secondary spark gaps being defined as to radial gap length G2 by the radial height of the sleeve projection (and possibly the axial position of extension element termination), one or more of the shell extension elements may also vary in real or effective thickness along their lengths to provide physical strength and/or specific radial gap length that accommodates significant clearance between insulating sleeve and shell part counterbore without requiring excessive height, and fragility, of the sleeve projection or projections.

Figures 7(a) to 7(h) each show schematically one half of the spark plug 100 or 200 in sectional elevation. Figure 7(a) corresponds to the view of Figure 2 (or Figure 3) in which the shell extension element is of uniform thickness T along its length to a termination 125ι in line with the ridge of the annular sleeve projection rib, so that secondary spark gap G2 is defined between the projection ridge and the end of the extension element with maximum exposure to the cylinder and fuel mixture beyond.

Figures 7(b) to 7(f) illustrate non-exhaustively a variety of thickness profiles in which the end of the extension element extends radially inwardly towards the sleeve projection to further shorten the secondary spark gap G2. Such thickness profiling is particularly suited to discrete, mounted extension elements 221₁ etc of plug 200 but may be defined by re-entrant machining or deforming of integral elements 121_! etc of plug 100 or a skirt wall of plug 400.

Figure 7(g) illustrates how the extension elements or equivalent skirt wall may be bent over to achieve the same effect.

Figure 7(h) shows an alternative arrangement whereby a somewhat longer shell part extension element 121'ι of uniform thickness T is used to mount a pin 127ι extending radially and defining with a sleeve projection 32 a secondary spark gap for the element. The pin may be mounted fixed to the surface or extending therethrough, possibly adjustable as to position for tuning the secondary spark gap length G2 during or after manufacture.

It will be appreciated that the variations of extension elements / opening spaces widths of Figure 6 and the extension element thickness variations of Figure 7 may be applied to extension elements formed by the skirt wall of Figure 5. Whereas in Figures 6 and 7 variations to circumferential widths and radial thicknesses of extension elements are shown separately, it will be appreciated that these may be combinations of individual elements. Furthermore, such elements may be formed whose circumferential width varies with thickness, that is, tapered radially such that the external wall is greater than the internal wall.

Although only a single form of sleeve projection is illustrated here it should be appreciated that any of the projection forms referred to in the aforementioned WO 03/043152 may be employed, whether as circumferentially complete (annular) rib or discrete projections, provided each projection effects a change in sleeve surface position relative to the ground conductor alongside that is relatively steep and conducive to effecting electric charge concentration in the vicinity of a secondary spark gap to be defined thereby. Furthermore, additional projections may be formed along the insulating sleeve which may combine with an extension element running alongside, one or more further secondary spark gaps of decreasing radial gap length.

Claims

1. A spark plug comprising an electrically insulating sleeve extending along a central axis of the plug, a first electrode mounted within the sleeve and having a tip extending beyond an end portion of the sleeve, a ground conductor, extending alongside the sleeve with a clearance therebetween, comprising a metallic shell part surrounding the sleeve axially spaced from the end portion of the sleeve to provide a circumferentially continuous shrouding thereof and a second electrode connected to the shell having at least one second electrode tip disposed adjacent the first electrode to define a primary spark gap, and at least one projection formed from electrically insulating material projecting laterally from the sleeve end portion so that the clearance between the sleeve and ground conductor alongside is locally reduced in an axially limited, electrical charge concentrating manner that effects formation of at least one secondary spark gap between the insulating sleeve and the ground conductor, the plug being characterised by said ground conductor extending axially from the circumferentially continuous shell part in a direction towards the end of the sleeve and providing in the vicinity of said sleeve projection or projections circumferentially alternate regions of ground conductor and lateral sleeve exposure opening, at least one of said ground conductor regions creating with at least one said sleeve projection a circumferentially limited secondary spark gap adjacent a said lateral sleeve exposing opening.

2. A spark plug as claimed in claim 1 in which the ground conductor extends axially as a plurality of shell part extension elements each of limited circumferential extent separated by spaces therebetween defining the lateral sleeve exposure openings.

3. A spark plug as claimed in claim 2 in which the opening spaces separating adjacent shell part extension elements are, at least in the vicinity of the sleeve projection or projections, of circumferential extent not less than the radial thickness of the adjacent shell part extension elements.

4. A spark plug as claimed in claim 2 or claim 3 in which the shell extension elements are, in a circumferential direction, arcuate and conform to the curvature of the circumferentially continuous shell part.

5. A spark plug as claimed in any one of claims 2 to 4 in which each shell part extension element is of substantially uniform circumferential width along it length axially.

6. A spark plug as claimed in any one of claims 2 to 4 in which in which each shell part extension element diminishes in circumferential width along its length axially.

7. A spark plug as claimed in any one of claims 2 to 6 in which each shell part extension element is of substantially uniform thickness.

8. A spark plug as claimed in any one of claims 2 to 6 in which at least one shell part extension element has an increased thickness region in the vicinity of a said sleeve projection and defining a secondary spark gap.

9. A spark plug as claimed in any one of claims 2 to 8 in which the shell extension elements extend substantially to the axial position of an associated sleeve projection.

10. A spark plug as claimed in any one of claims 2 to 9 in which the shell extension part elements extend beyond the axial position of an associated sleeve projection such that a second spark gap is defined between the peak of the projection and opening space bounding edge of each extension element.

11. A spark plug as claimed in any one of claims 2 to 10 in which the extension part elements are of substantially uniform length.

12. A spark plug as claimed in any one of claims 2 to 11 in which in which at least one shell part extension element of the shell part comprises a discrete member mounted to the end of the shell part.

13. A spark plug as claimed in any one of claims 2 to 11 in which at least one shell part extension element of the ground conductor is integral with the shell part.

14. A spark plug as claimed in any one of claims 2 to 12 in which said at least one shell part extension element is of a different metal than the shell part.

15. A spark plug as claimed in any one of claims 2 to 14 in which each shell part extension element is unsupported at its end remote from the shell part.

16. A spark plug as claimed in any one of claims 2 to 14 in which each shell part extension element is supported relative to at least one other at its end remote from the shell part.

17. A spark plug as claimed in claim 16 in which the ground conductor extends axially from the circumferentially continuous shell part as a circumferentially complete skirt wall extending axially beyond said sleeve projection or projections, said skirt wall containing axially and circumferentially bounded through apertures in the vicinity of said sleeve projection or projections forming said lateral sleeve exposing openings and between adjacent openings said shell part extension elements.

18. A spark plug as claimed in any one of the preceding claims in which the second electrode is mounted on the ground electrode at the end of the circumferentially continuous shell part and extends alongside the sleeve to the end thereof and defines with a said sleeve projection the secondary spark gap.

19. A spark plug as claimed in any one of claims 2 to 17 in which the second electrode is mounted to an end of a said shell part extension element.

20. A spark plug as claimed in any one of claims 2 to 17 in which the second electrode is integral with a said shell part extension element.

21. A spark plug as claimed in any one of the preceding claims in which each sleeve projection effects a steep change in position of the sleeve surface relative to an adjacent ground conductor part.

22. A spark plug as claimed in claim 21 in which each sleeve projection effects a steep sided circumferentially extending ridge of small axial extent.

23. A spark plug as claimed in claim 21 or claim 22 in which the secondary spark gap is defined by said one or more sleeve projections at a substantially fixed distance from the end of the sleeve.

24. A spark plug as claimed in claim 23 when dependent on claim 22 in which the sleeve projection closest to the end of the sleeve is defined by an annular rib extending circumferentially around the sleeve.